This invention relates to the fabrication of lithium thin film secondary batteries.
Batteries are galvanic electrochemical cells which store and supply electrical energy as a product of a chemical reaction. In their simplest conceptualization, batteries have two electrodes, one that supplies electrons by virtue of an oxidation process occurring at that electrode, termed the anode (hereinafter, xe2x80x9canodic processesxe2x80x9d), and a second one that consumes electrons by virtue of a reduction process occurring at that electrode, termed the cathode (hereinafter, xe2x80x9ccathodic processesxe2x80x9d).
There are two broad classifications of batteries, primary batteries and secondary batteries. In primary batteries, either the anodic process, or the cathodic process, or both are irreversible, as defined for electrochemical processes. For this reason, once the reagents participating in the reactions are by-and-large consumed, the battery can""t be returned to a charged state by electrochemical means.
In secondary batteries the electron producing and consuming reactions are for the most part reversible, as defined for electrochemical processes, and therefore such a battery can be cycled between a charged and discharged state electrochemically.
The reactions employed in batteries to produce and consume electrons are redox reactions. A pair of such reactions is called a redox couple. Each redox reaction is termed a half cell, with two half cells constituting a simple battery when the half cells are placed in ionic communication such that voltage potential appears between the electrodes of the half cells. Typically, the electrodes of several sets of half cells are electrically coupled together in either series or parallel configuration to supply a greater voltage or a greater current, or both than that which is available from a single set of half cells.
The voltage potential of a simple battery (a single set of half cells) is fixed by the set of redox couples chosen to produce and consume electrons. The redox couples are chosen such that the potential energy of the electron producing reaction yields electrons of sufficient potential energy to supply electrons to the electron consuming reaction. The electromotive force (emf) supplied by the battery is the difference between the potential energy of the electrons produced by the electron producing reaction and that required of the electrons consumed by the electron consuming reaction. As electrons are transferred from the electron producing reaction to the electron consuming reaction, charge within the half cells in which these reactions are carried out is balanced by the movement of ions between the half cells.
Ion batteries utilize materials in their construction that exhibit low resistance to ion movement through and within their structure. Thus, ion batteries improve the efficiency of storing and transferring electrical energy by reducing the resistance that ions must overcome at the interfaces of the various phases within the battery, and improve energy storage capacity by utilizing materials which do not polarize, and therefore during charge movement do not build up space charge regions which contribute resistance to charge movement within the battery. This feature tends to permit a higher density of charge species to be moved within a given volume of an ion battery than is possible with conventional materials. Additionally, thin film techniques permit the formation of very thin electrolyte layers separating the redox couples, further reducing resistance to charge movement within the battery structure. Thin film ion batteries hold the promise of much higher energy densities than are possible from conventional wet chemistry batteries.
Ion batteries can be prepared from macroscopic compounding techniques to fabricate anode, cathode, and electrolyte materials which are then bonded together to form the battery (the so called xe2x80x9cthick filmxe2x80x9d technique), or by depositing thin films of such materials using vacuum techniques, producing xe2x80x9cthin filmxe2x80x9d batteries. The fabrication of batteries by xe2x80x9cthick filmxe2x80x9d techniques is usually directed toward high current capacity devices. Thin film batteries are generally employed in low current draw applications in which space and weight must be conserved.
U.S. Pat. No. 5,895,731 (hereinafter xe2x80x9cthe ""731 patent) to Clingempeel is exemplary of batteries fabricated using xe2x80x9cthick filmxe2x80x9d construction. The ""731 patent teaches the preparation of a cathode from a mixture of powders of titanium nitride, selenium, silicon, and buckminsterfullerene bonded together with epoxy polymer to aluminum foil. Additionally the ""731 patent teaches the preparation of an anode from lithium foil, fiberglass matting and n-methyl-pyrrollidone, and the preparation of an electrolyte layer by gelation of a mixture of n-methyl-pyrrollidone, lithium metal, and polyimide powder to produce a cross-linked lithium gel electrolyte which is cast into a sheet. These materials are pressed together and sealed in polyimide plastic with appropriate electrical contacts to the anode and cathode. Production of such a battery requires strict atmospheric control during fabrication to exclude moisture and oxygen, and numerous preparatory steps.
Thin film battery fabrication techniques are well known to those skilled in the art. Thus, for example, U.S. Pat. No. 5,338,625 to Bates (hereinafter xe2x80x9cthe ""625 patentxe2x80x9d), teaches the formation of a lithium based thin film battery by vacuum deposition of two co-planar vanadium current collectors on an insulating substrate. Upon one of the current collectors is deposited a cathode comprising an amorphous vanadium oxide layer. This cathode layer is deposited by reactive ion sputtering from a vanadium target in an oxygen environment. On top of the cathode layer is deposited an amorphous lithium phosphorous oxynitride (also called xe2x80x9cSub-stoichiometric lithium phosphorous oxynitridexe2x80x9d) layer which acts as an electrolyte. This layer is deposited by reactive ion sputtering of lithium orthophosphate in a nitrogen atmosphere. Finally, a layer of lithium metal was vacuum evaporated onto the assembly, covering both the bare current collector and the current collector bearing the cathode and electrolyte. The disclosed thin film battery contains a bare lithium anode, and as such requires further steps to isolate the anode from the ambient environment. Additionally, because of the presence of the relatively low melting lithium metal the disclosed battery has low tolerance for heating.
Hybrid batteries containing a combination of elements prepared by macroscopic compounding techniques which in turn have thin films deposited onto them have also been described. Thus, U.S. Pat. Nos. 5,569,520 (hereinafter xe2x80x9cthe ""520 patentxe2x80x9d)and 5,612,152 (hereinafter xe2x80x9cthe ""152 patent xe2x80x9d), both to Bates, describe a preparation of a lithium manganate cathode pellet using traditional ceramic processing techniques (e.g., hot pressing and sintering the powder). The pellet is then subjected to deposition of a thin electrolyte film of, e.g., lithium phosphorous oxynitride (Sub-stoichiometric lithium phosphorous oxynitride), by reactive ion sputtering using the techniques described above for the ""625 patent to Bates. A lithium film anode is then deposited on the exposed face of the electrolyte film, again by vacuum techniques, forming a multilayered thin film battery. The ""520 and ""152 patents further disclose that an additional mass of lithium can be incorporated into the battery by sandwiching the anode of the multi-layered battery material described above with an additional sheet of lithium foil and cycling the sandwiched construction through several charging/discharging cycles. In this process, the thin lithium film is xe2x80x9cplatedxe2x80x9d onto the foil sandwiched with it to form a continuous phase with the electrolyte/lithium metal interface, bonding the lithium foil into the multi-layered material.
The ""152 and ""520 patents further disclose that deposition of a lithium anode film on the exposed face of the electrolyte of a multi-layer battery material can be eliminated for the process of bonding a foil sandwiched to the multi-layer battery material. These patents disclose that pressing a piece of lithium foil against the exposed face of the electrolyte layer of the multi-layer battery material and cycling the battery between charged and discharged states will also bond the lithium foil to the multi-layer battery material by virtue of deposition of lithium metal from the electrolyte during battery charging onto the face of the lithium foil in contact with the electrolyte.
Finally, the ""152 and ""520 patents teach that deposition of an anode can be dispensed with. Batteries can be fabricated by vacuum application of an electrolyte film onto a cathode material and the application of a current collector onto the exposed side of the electrolyte film. Cycling the battery through a charge cycle electrochemically deposits a lithium anode layer between the current collector and the electrolyte. Thus, a thin film of Sub-stoichiometric lithium phosphorous oxynitride was deposited by vacuum evaporation onto a Li2MnO4 cathode pellet, forming a Sub-stoichiometric lithium phosphorous oxynitride film coating on one face of the cathode. Onto the exposed face of the Sub-stoichiometric lithium phosphorous oxynitride film coating a current collecting layer of vanadium metal was deposited. This multi-layer battery material was subjected to a charging current, whereupon lithium metal was extracted from the electrolyte layer and plated onto the face of the vanadium current collector in contact with the electrolyte film.
Additional disclosure of the technique of electrochemical deposition of a lithium metal anode within the multi-layer structure of an electrolyte and cathode material has been described in PCT application US00/06997 of Lockheed Martin Energy Research Corporation, filed 17 Mar. 2000 (hereinafter, xe2x80x9cthe ""997 applicationxe2x80x9d). This application teaches the formation of a multi-layer battery material by sequential deposition of various thin films onto an insulating substrate. In this manner, a cathode current collector in the form of an Ag or Pt thin film was first deposited onto an alumina substrate. Following this a cathode film of Li2MnO4 was deposited onto the current collector by vacuum sputtering techniques. Onto the cathode film was deposited an electrolyte thin film of Sub-stoichiometric lithium phosphorous oxynitride by reactive ion sputtering. Onto the exposed face of the Sub-stoichiometric lithium phosphorous oxynitride electrolyte film was deposited a metal thin film to serve as an anode current collector. The metal was selected from metals that do not form intermetallic compounds with lithium, generally the group 8 transition metals, Ti, aluminum, gold, and in particular the refractory metals, as will be known to one skilled in the art.
Thus fabricated, this multi-layer battery material was subject to a charging current whereby a lithium anode was plated between the current collector thin film and the electrolyte. The ""997 application further teaches that a protective layer must be deposited onto the current collector for the electrochemical anode deposition/stripping to be reversible. In this role, deposition of films of lithium nitride or Sub-stoichiometric lithium phosphorous oxynitride onto the exposed face of the anode current collector film as protective layers is taught. The ""997 application discloses that this over-layer functions to prevent lithium chemical attack upon the current collector, prevent undesirable morphology from occurring in the deposited lithium layer (a so called xe2x80x9cfluffyxe2x80x9d or xe2x80x9cmossyxe2x80x9d morphology), and to absorb the volume change thought to accompany the deposition of the lithium metal layer. The over-layer is said to additionally impart electrical insulation, mechanical protection, and act as a barrier to moisture and oxygen for the lithium layer.
While the plated lithium anode prior art has addressed some of the problems associated with the Li/LiMxOy couple (where M=a transition metal), such as the heat sensitivity of lithium metal and some of the difficulties due to the air sensitive nature of lithium (see U.S. Pat. No. 5,871,865 to Barker et. al. for a discussion of these and other problems arising from the presence of lithium metal in the preparation of batteries) there is still some inherent instability in lithium based batteries constructed according to disclosures in the prior art. This instability can be addressed by the addition of a protective layer to the anode current collector. Such a solution increases the bulk of a battery, reducing its current density, and adds a processing step, increasing its cost, without increasing the net capacity or performance of the battery.
The process of the present invention for production of a multi-layer thin film battery precursor structure is directed to eliminating the need for an additional protective layer applied to the anode or anode current collector and to increasing the amount of lithium that may be electrochemically formed as an anode during activation of an xe2x80x9canodelessxe2x80x9d battery precursor in the manner of Bates.
The present invention is directed toward minimizing the number of processing steps required to fabricate a thin film battery, and at increasing charge retention in a battery and the number of charge/discharge cycles that a battery can be subjected to without significant degradation. Additonally, the present invention seeks to provide a method of producing a lithium based battery which is air stable without the application of a protective overlayer following the formation of the anode, cathode, and electrolyte layers and charging of such a battery.
One aspect of the present invention is a process of producing a secondary, lithium based, thin film battery, having the steps of:
a) depositing a film comprising a solid state electrolyte material that is a conductor of lithium ions onto an exposed, conductive face of a substrate;
b) depositing a film of a transition metal oxide onto the electrolyte material;
c) forming a cathode film layer by lithiating the transition metal oxide film until it contains a supra-stoichiometric amount of lithium;
d) depositing an electron-conductive current collector film upon the cathode film layer;
e) forming a lithium metal buried anode layer between the conductive face of the substrate and the solid state electrolyte material using a flowing current between the substrate conductive face and the cathode current collector, in the process oxidizing the cathode film layer and causing lithium ions to migrate into and through the solid state electrolyte material, and then to be reduced to lithium metal and forming said buried anode layer; and
f) maintaining the current flow until the buried anode layer contains a desired amount of lithium metal.
Another aspect of the present invention are lithium thin film batteries with buried anodes and reverse structures made according to the above process.
Another aspect of the present invention is a process for producing a lithium based, thin film battery precursor composite structure, comprising the steps of:
a) depositing a film comprising a solid state electrolyte material that is a conductor of lithium ions onto an exposed, conductive face of a substrate;
b) depositing a film comprising a transition metal oxide on top of the film of solid state electrolyte material;
c) forming a cathode film layer by lithiating the transition metal oxide film until it contains a supra-stoichiometric amount of lithium; and
d) depositing a current collector film upon an exposed face of said cathode film layer, said current collector comprising an electron conducting material.
Another aspect of the present invention are lithium battery precusor composite structures made according to the process for producing battery precursor composite structures recited above.
Another aspect of the present invention is a lithium battery composite precursor, characterized by its ability to form a buried lithium anode layer at the interface between an anode current collector and an electrolyte when a current is maintained between the anode current collector and the cathode current collector, and its ability to be chemically stable when exposed to an ambient environment, the precursor having an anode current collector layer that forms a support and has at least one conductive face; an electrolyte layer that is a conductor of lithium ions and has one face in communication with a conductive face of the anode current collector layer; a cathode layer that is in communication with a face of the electrolyte layer that is not in communication with the anode current collector layer; and a cathode current collector layer that is in communication with a face of the cathode layer that is not in communication with the electrolyte layer.
Another aspect of the present invention is a lithium thin film battery having an anode current collector layer that forms a support and has at least one conductive face; a buried anode layer comprising lithium metal in communication with a conductive face of said anode current collector; an electrolyte that is a conductor of lithium ions and is in communication with said anode layer; a cathode layer that is in communication with a face of said electrolyte layer that is not in communication with said anode layer; and a cathode current collector layer that is in communication with a face of the cathode layer that is not in communication with the electrolyte layer, the battery being characterized by an increase in the amount of metallic lithium contained in its buried anode layer upon charging and a reduction in the amount of lithium metal in its buried anode layer upon discharging, and its chemical stability when exposed to an ambient environment in any state of charge.
Other aspects of this invention will appear from the following description and appended claims, reference being made to the accompanying drawings forming a part of this specification wherein like reference characters designate corresponding parts in the several views.